1. Field of the Invention
The present invention relates to microelectromechanical ("MEM") structures and particularly relates to methods to inhibit adhesion of micromachined structures to a substrate (or to adjacent structures) after release etch processing. The present invention more particularly relates to methods to inhibit adhesion of MEM structures by providing a particular shape to the MEM structures or by incorporating a cleft between a portion of the MEM structures and adjacent field regions. The present invention also more particularly relates to elevated temperature post-release etch treatments that inhibit adhesion of MEM structures.
2. Description of the Invention Background
A common step in the fabrication of MEM structures, more generally referred to herein as "microstructures", is a wet etching step to release a portion of the structures from a substrate and create a "suspended" microstructure having a void or gap between the released portion of the microstructure and the substrate and one or more rigid base regions attached to the substrate. The released portion is typically a beam or plate having top and bottom surfaces which are suspended to be substantially parallel with the surface of the substrate. Common suspended MEM structures include cantilevered beams, double supported beams and plates suspended above a substrate by four supports. Devices which incorporate such suspended MEM structures include accelerometers, pressure sensors, flow sensors, transducers, microactuators, and electrostatic comb drives. As used herein, the term microstructure refers to a structure fabricated by MEM manufacturing techniques, which include photolithography, thin film deposition, bulk micromachining, surface micromachining, and etching.
The release etch can be an anisotropic etch to either produce a cavity in the substrate ("bulk micromachining") or remove a sacrificial layer intermediate a portion of the microstructure and the substrate ("surface micromachining"). In both micromachining processes, the released portion of the microstructures often permanently adhere to the substrate or adjacent structures after post-etch rinsing and drying procedures and this reduces the micromachining process yield. The microstructure adhesion phenomenon is known as stiction and is referred to herein by that term. The significance of stiction has increased as more compliant MEM structures have been designed and fabricated.
Although stiction can occur between a microstructure and a substrate or any adjacent structure, for purposes of simplification, the present specification only refers to stiction occurring between a microstructure and a substrate. It is to be understood that the following discussion is equally applicable to stiction which may occur between a microstructure and an adjacent non-substrate structure, for example, an adjacent microstructure. It is believed that the inventions described herein are equally useful for preventing stiction between the released portion of suspended microstructures and adjacent non-substrate structures.
The mechanism by which stiction occurs remains a matter of dispute, but can be divided into two stages: (a) mechanical collapse of the released portion of the microstructure to contact or move very close to the substrate and (b) adhesion of the released portion of the microstructure to the substrate. It is believed that the microstructure's mechanical collapse is initiated by high surface tension forces resulting from etchant rinse liquid trapped in the capillary-like spaces between the microstructure and the substrate. Several mechanisms have been proposed to explain the adhesion of the microstructure to the substrate, including solid bridging, liquid bridging, Van der Waals forces, and hydrogen bridging. See R. L. Alley et al., Proc. IEEE Solid-State Sensor & Actuator Workshop, Hilton Head Island, S.C., pp. 202-207 (1992); R. Legtenberg et al., "Stiction of Surface Micromachined Structures After Rinsing and Drying: Model and Investigation of Adhesion Mechanisms", Sensors and Actuators A, 43 (1994) pp. 230-238. Mastrangelo and Hsu provide a detailed discussion of the adhesion of suspended surface micromachined structures in the papers "Mechanical Stability and Adhesion of Microstructures Under Capillary Forces--Part I: Basic Theory" and "Mechanical Stability and Adhesion of Microstructures Under Capillary Forces--Part II: Experiments", J. of Microelectromechanical Systems, Vol. 2, No. 1 (March 1993), pp. 33-55, both of which are hereby incorporated herein by reference.
A number of techniques have been developed to avoid stiction. One technique is to reduce the real contact area between the released portion of the microstructure and the underlying substrate either through nanoscale roughness intrinsic to one or both surfaces or through the formation of microscale standoff bumps or "dimples" on the microstructure. Reducing the real contact area between the microstructure and the substrate weakens adhesive forces therebetween and reduces the likelihood that the microstructure will permanently adhere to the substrate.
Another group of stiction inhibition techniques eliminates the source of surface tension between the released portion of microstructure and the substrate and prevents the microstructure's initial collapse by eliminating the gas-liquid interface. In one such technique, the etchant rinse liquid is solidified and then removed by sublimation. D. Kobyashi et al., Proc. IEEE Micro Electro Mechanical Systems, Travemunde, Germany 1992 (IEEE, New York, 1992) p. 214; N. Takeshima et al., Proc. Int. Conf. Solid-State Sensors & Actuators (Transducers '91), San Francisco, Calif., 1991 (IEEE, New York, 1991) p. 198; Guckel et al., "Fabrication of Micromechanical Devices from Polysilicon Films With Smooth Surfaces", Sensors & Actuators, 20 (1989) p. 117-22. In an alternate procedure, using a pressure-controlled and temperature-controlled chamber, the rinse liquid is evacuated in its supercritical state, wherein the gas-liquid distinction does not exist. G. T. Mulhern et al., Proc. Int. Conf. Solid State Sensors & Actuators (Transducers '93), Yokohama, 1993 (IEEJ, Tokyo, 1993) p. 296.
Techniques have also been developed that utilize temporary support structures to mechanically reinforce the released portion of the microstructure and retain the desired microstructure/substrate gap during wet etching, rinsing and drying. One method employs tethers which are broken after fabrication of the microstructure. See K. Minami et al., Proc. Micro Electro Mechanical Systems, Fort Lauderdale, Fla., 1993 (IEEE, New York, 1993) p. 53. In an alternate method, temporary supports are incorporated into the microstructure and are subsequently removed; after release etching and rinsing, the rinse liquid is replaced with a photoresist and acetone mixture which is dried and supports the microstructure above the substrate during rinse liquid drying. The support structures are subsequently ashed in oxygen plasma. See M. Orpana et al., "Control of Residual Stress of Polysilicon Thin Films By Heavy
In Surface Micromachining", Proc. Int. Conf. Solid-State Sensors & Actuators (Transducers '91), San Francisco, Calif., 1991 (IEEE, New York, 1991) p. 957-60. A third alternative procedure utilizing temporary columns is the subject of U.S. Pat. No. 5,258,097 to Mastrangelo and utilizes an array of discrete microscopic polymer columns, preferably of xylylene, which provide additional mechanical support to surface micromachined structures during the sacrificial etch to prevent collapse of the microstructures. The polymer columns are removed after the rising and drying steps by oxygen plasma etching. See also C. H. Mastrangelo et al., "A Dry-Release Method Based on Polymer Columns for Microstructure Fabrication", Proc. IEEE Micro Electro Mechanical Systems, Fort Lauderdale, Fla., 1993, (IEEE, New York, 1993) pp. 77-81.
Each of the above methods for inhibiting stiction has significant drawbacks. All of the above methods suffer from yield loss, are process-oriented, and incorporate additional steps into the microstructure fabrication process, thereby increasing the complexity and cost of fabrication. Both the sublimation and supercritical drying methods require specialized equipment to dry the rinse liquid as compared with conventional fabrication techniques wherein the rinse liquid is evaporated at ambient (i.e., room) temperature. Both the dimple method and each of the above temporary support methods require additional fabrication steps prior to the rinse liquid drying step, thereby significantly increasing microstructure fabrication time and adding considerable additional cost per microstructure. In particular, in the temporary support method utilizing a photoresist/acetone mixture, because the microstructures have already been released from the substrate when the rinse liquid is replaced by the mixture, the mixture must be added very carefully to avoid damaging the microstructures, especially if the microstructures are relatively complaint. Also, several fabrication steps, including an additional masking step, are necessary to form the polymer columns of the method of U.S. Pat. No. 5,258,097, and a final plasma etching step must be used to remove the polymer columns and free the microstructure.
An additional disadvantage of both the temporary support methods and the gas-liquid interface elimination methods is that those methods may only reduce stiction during microstructure fabrication and the methods do not inhibit stiction that can occur after fabrication when the microstructure is subjected to environments including liquid or liquid vapor. Also, none of the above methods prevent the build-up of residue at positions on the microstructure where stiction may occur.
It is apparent from the aforementioned disadvantages of the known methods that a need exists for a method of inhibiting stiction of suspended microstructures that does not significantly increase the complexity of conventional MEM structure fabrication techniques, can be carried out without specialized equipment, and does not significantly add to the cost of or time required for fabrication.